Cellular radiosensitivity and DNA damage in primary human fibroblasts

Chromosomal Rearrangements and Chromothripsis: The Alternative End Generation Model

Chromothripsis defines a genetic phenomenon where up to hundreds of clustered chromosomal rearrangements can arise in a single catastrophic event. The phenomenon is associated with cancer and congenital diseases. Most current models on the origin of chromothripsis suggest that prior to chromatin reshuffling numerous DNA double-strand breaks (DSBs) have to exist, i.e., chromosomal shattering precedes rearrangements. However, the preference of a DNA end to rearrange in a proximal accessible region led us to propose chromothripsis as the reaction product of successive chromatin rearrangements. We previously coined this process Alternative End Generation (AEG), where a single DSB with a repair-blocking end initiates a domino effect of rearrangements. Accordingly, chromothripsis is the end product of this domino reaction taking place in a single catastrophic event.

Hypoxanthine Reduces Radiation Damage in Vascular Endothelial Cells and Mouse Skin by Enhancing ATP Production via the Salvage Pathway

An effective method that can protect radiation-damaged tissues from apoptosis and promote tissue repair has not been reported to date. Hypoxanthine (Hx) is an intermediate metabolite in the purine degradation system that serves as a substrate for ATP synthesis via the salvage pathway. In this study, we focused on the transient decrease in intracellular ATP concentration after radiation exposure and examined the protective effect of Hx against radiation-induced tissue damage. Human umbilical vein endothelial cells were X irradiated, and the cell viability and incidence of apoptosis and DNA double-strand breaks (DSBs) were evaluated at different Hx concentrations. We found that in the presence of 2-100 μM Hx, the percentages of DSBs and apoptotic cells after 2, 6 and 10 Gy dose of radiation significantly decreased, whereas cell viability increased in a concentration-dependent manner. Moreover, the addition of Hx increased the levels of AMP, ADP, and ATP in the cells at 2 h postirradiation, suggesting that Hx was used for adenine nucleotide synthesis through the salvage pathway. Administration of a xanthine oxidoreductase inhibitor to a mouse model of radiation dermatitis resulted in increased blood Hx levels that inhibited severe dermatitis and accelerated recovery. In conclusion, the findings provide evidence that increasing the levels of Hx to replenish ATP could be an effective strategy to reduce radiation-induced tissue damage and elucidating the detailed mechanisms underlying the protective effects of Hx could help develop new protective strategies against radiation.

Radiation-Induced Lung Injury (RILI)

Radiation pneumonitis (RP) and radiation fibrosis (RF) are two dose-limiting toxicities of radiotherapy (RT), especially for lung, and esophageal cancer. It occurs in 5-20% of patients and limits the maximum dose that can be delivered, reducing tumor control probability (TCP) and may lead to dyspnea, lung fibrosis, and impaired quality of life. Both physical and biological factors determine the normal tissue complication probability (NTCP) by Radiotherapy. A better understanding of the pathophysiological sequence of radiation-induced lung injury (RILI) and the intrinsic, environmental and treatment-related factors may aid in the prevention, and better management of radiation-induced lung damage. In this review, we summarize our current understanding of the pathological and molecular consequences of lung exposure to ionizing radiation, and pharmaceutical interventions that may be beneficial in the prevention or curtailment of RILI, and therefore enable a more durable therapeutic tumor response.

Radiosensitivity and relative biological effectiveness based on a generalized target model

By considering both cellular repair effects and indirect effects of radiation, we have generalized the traditional target model, and made it have a linear-quadratic-linear characteristic. To assess the repair capacity-dependent radiosensitivity and relative biological effectiveness (RBE), the generalized target model was used to fit the survival of human normal embryonic lung fibroblast MRC-5 cells in the G0 and G1 phases after various types of radiations. The fitting results indicate that the generalized target model works well in the dose ranges considered. The resulting calculations qualitatively show that the parameter ratio (a/V) in the model could represent the cellular repair capacity. In particular, the significant linear correlations between radiosensitivity/RBE and cellular repair capacity are observed for different slopes of the linear regression curves. These results show that the radiosensitivity and RBE depend on the cellular repair capacity and can be regulated by linear energy transfer. These analyses suggest that the ratio a/V in the generalized target model can also be used for radiation damage assessment in radiotherapy.

The yield of DNA double strand breaks determined after exclusion of those forming from heat-labile lesions predicts tumor cell radiosensitivity to killing

BACKGROUND AND PURPOSE

The radiosensitivity to killing of tumor cells and in-field normal tissue are key determinants of radiotherapy response. In vitro radiosensitivity of tumor- and normal-tissue-derived cells often predicts radiation response, but high determination cost in time and resources compromise utility as routine response-predictor. Efforts to use induction or repair of DNA double-strand-breaks (DSBs) as surrogate-predictors of cell radiosensitivity to killing have met with limited success. Here, we re-visit this issue encouraged by our recent observations that ionizing radiation (IR) induces not only promptly-forming DSBs (prDSBs), but also DSBs developing after irradiation from the conversion to breaks of thermally-labile sugar-lesions (tlDSBs).

MATERIALS AND METHODS

We employ pulsed-field gel-electrophoresis and flow-cytometry protocols to measure total DSBs (tDSB=prDSB+tlDSBs) and prDSBs, as well as γH2AX and parameters of chromatin structure.

RESULTS

We report a fully unexpected and in many ways unprecedented correlation between yield of prDSBs and radiosensitivity to killing in a battery of ten tumor cell lines that is not matched by yields of tDSBs or γH2AX, and cannot be explained by simple parameters of chromatin structure.

CONCLUSIONS

We propose the introduction of prDSBs-yield as a novel and powerful surrogate-predictor of cell radiosensitivity to killing with potential for clinical application.

DNA damage at respiratory distress, but not acute time-points, correlates with tissue fibrosis following thoracic radiation exposure in mice

PURPOSE

Radiation exposure can result in DNA damage but whether the extent of DNA damage correlates with the radiation-induced tissue injury in the lung is not known. We aimed to determine whether numbers of γH2AX foci, representing histone H2AX phosphorylation a marker of DNA damage, measured within days of radiation exposure, correlated with known later lung injury responses in eight inbred mouse strains.

MATERIALS AND METHODS

Mice received 18 Gy pulmonary irradiation and numbers of γH2AX positive nuclei in the lung were immunohistochemically determined.

RESULTS

Numbers of γH2AX foci, assessed up to seven days post irradiation did not correlate with pulmonary fibrosis. γH2AX counts from mice in respiratory distress, however, significantly correlated with fibrosis and lungs from mice treated with a fibrosis-reducing antagonist had fewer γH2AX foci.

CONCLUSIONS

Acute response measures of pulmonary DNA damage did not predict for pathology, but levels of this marker in distressed mice were correlative of fibrosis.

Misrepair of DNA double-strand breaks in patient with unidentified chromosomal fragility syndrome and family history of radiosensitivity

PURPOSE

To test the hypothesis that differences in DNA double-strand breaks (DSB) repair fidelity underlies differences in radiosensitivity.

MATERIALS AND METHODS

A primary fibroblast culture (C42) derived from a pediatric cancer patient treated with reduced radiation doses consequent to a family history of radiosensitivity reminiscent of chromosomal fragility syndrome, was compared to a normal control (C29). DNA DSB rejoining and repair fidelity were studied by Southern blotting and hybridization to specific fragments: Alu repetitive sequence representing the overall DSB rejoining capacity in the genome and a 3.2 Mbp NotI restriction fragment on chromosome 21 for DSB repair fidelity.

RESULTS

Although both assays showed statistically significant difference (p ≤ 0.05) between the two cell strains in residual misrepaired (un-or mis-rejoined) DSB (24 h after 30 or 80 Gy), the residual damage was lower in the Alu enriched genome assay compared to NotI assay (0.01-0.07 and 0.10-0.37, respectively).

CONCLUSIONS

These results suggest that, in comparison to classic DSB repair experiment, an assay of measuring DNA DSB repair fidelity can provide better resolution and a more accurate estimate of misrepair of radiation-induced DNA damage, which underlies genomic instability and increased radiosensitivity.

Defining molecular and cellular responses after low and high linear energy transfer radiations to develop biomarkers of carcinogenic risk or therapeutic outcome

The variability in radiosensitivity across the human population is governed in part by genetic factors. The ability to predict therapeutic response, identify individuals at greatest risk for adverse clinical responses after therapeutic radiation doses, or identify individuals at high risk for carcinogenesis from environmental or medical radiation exposures has a medical and economic impact on both the individual and society at large. As radiotherapy incorporates particles, particularly particles larger than protons, into therapy, the need for such discriminators, (i.e., biomarkers) will become ever more important. Cellular assays for survival, DNA repair, or chromatid/chromosomal analysis have been used to identify at-risk individuals, but they are not clinically applicable. Newer approaches, such as genome-wide analysis of gene expression or single nucleotide polymorphisms and small copy number variations within chromosomes, are examples of technologies being applied to the discovery process. Gene expression analysis of primary or immortalized human cells suggests that there are distinct gene expression patterns associated with radiation exposure to both low and high linear energy transfer radiations and that those most radiosensitive are discernible by their basal gene expression patterns. However, because the genetic alterations that drive radio response may be subtle and cumulative, the need for large sample sizes of specific cell or tissue types is required. A systems biology approach will ultimately be necessary. Potential biomarkers from cell lines or animal models will require validation in a human setting where possible and before being considered as a credible biomarker some understanding of the molecular mechanism is necessary.

Residual DNA and chromosomal damage in ex vivo irradiated blood lymphocytes correlated with late normal tissue response to breast radiotherapy

PURPOSE

To test the association of DNA double-strand break (DSB) repair and chromosomal radiosensitivity in ex vivo irradiated blood lymphocytes with late-onset normal tissue responses following breast radiotherapy.

METHODS

Breast cancer patients with minimal (controls) or marked late radiotherapy changes (cases) were retrospectively selected. DSB were quantified by γH2AX/53BP1 immunofluorescence microscopy 0.5 and 24 h after exposure of unstimulated blood lymphocytes to 0.5 and 4 Gy X-rays, respectively. Chromosomal aberrations were scored in blood lymphocyte metaphases after 6 Gy X-rays.

RESULTS

Despite similar foci levels at 0.5 h in cases (n=7) and controls (n=7), foci levels 24 h after 4 Gy irradiation differed significantly between them (foci per cell were 12.8 in cases versus 10.2 in controls, p=0.004). Increased chromosomal radiosensitivity was also observed in cases (aberrations per cell were 5.84 in cases versus 3.79 in controls, p=0.001) with exchange and deletion type aberrations contributing equally to the difference between cases and controls. Residual foci correlated with formation of deletions (Spearman's R=0.589, p=0.027) but not exchanges (R=0.367, p=0.197) in blood lymphocytes from the same patients.

CONCLUSIONS

Higher levels of exchange type aberrations observed among radiosensitive breast cancer patients suggest a role for DSB misrepair, in addition to residual damage, as determinants of late normal tissue damage. Correlation of residual foci levels with deletion type aberration yields in the same cohort confirms their mechanistic linkage.

H2AX phosphorylation screen of cells from radiosensitive cancer patients reveals a novel DNA double-strand break repair cellular phenotype

Background:

About 1–5% of cancer patients suffer from significant normal tissue reactions as a result of radiotherapy (RT). It is not possible at this time to predict how most patients’ normal tissues will respond to RT. DNA repair dysfunction is implicated in sensitivity to RT particularly in genes that mediate the repair of DNA double-strand breaks (DSBs). Phosphorylation of histone H2AX (phosphorylated molecules are known as γH2AX) occurs rapidly in response to DNA DSBs, and, among its other roles, contributes to repair protein recruitment to these damaged sites. Mammalian cell lines have also been crucial in facilitating the successful cloning of many DNA DSB repair genes; yet, very few mutant cell lines exist for non-syndromic clinical radiosensitivity (RS).

Methods:

Here, we survey DNA DSB induction and repair in whole cells from RS patients, as revealed by γH2AX foci assays, as potential predictive markers of clinical radiation response.

Results:

With one exception, both DNA focus induction and repair in cell lines from RS patients were comparable with controls. Using γH2AX foci assays, we identified a RS cancer patient cell line with a novel ionising radiation-induced DNA DSB repair defect; these data were confirmed by an independent DNA DSB repair assay.

Conclusion:

γH2AX focus measurement has limited scope as a pre-RT predictive assay in lymphoblast cell lines from RT patients; however, the assay can successfully identify novel DNA DSB repair-defective patient cell lines, thus potentially facilitating the discovery of novel constitutional contributions to clinical RS.

The difference in LET and ion species dependence for induction of initially measured and non-rejoined chromatin breaks in normal human fibroblasts

We studied the LET and ion species dependence of the induction of chromatin breaks measured immediately after irradiation as initially measured breaks and after 24 h postirradiation incubation (37 degrees C) as non-rejoined breaks in normal human fibroblasts with different heavy ions, such as carbon, neon, silicon and iron, generated by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Science (NIRS). Chromatin breaks were measured as an excess number of fragments of prematurely condensed chromosomes using premature chromosome condensation (PCC). The results showed that the number of excess fragments per cell per Gy for initially measured chromatin breaks was dependent on LET in the range from 13.3 to 113.1 keV/mum but was not dependent on ion species. On the other hand, the number of non-rejoined chromatin breaks detected after 24 h postirradiation incubation was clearly dependent on both LET and ion species. No significant difference was observed in the cross section for initially measured breaks, but a statistically significant difference was observed in the cross section for non-rejoined breaks among carbon, neon, silicon and iron ions. This suggests that the LET-dependent structure in the biological effects is reflected in biological consequences of repair processes.

Influence of the TP53 codon 72 polymorphism on the cellular responses to X-irradiation in fibroblasts from nonagenarians

In mice, genetic modification of the gene encoding p53 affects both cancer incidence and longevity. In humans, we recently found that a TP53 codon 72 Arginine (Arg) to Proline (Pro) polymorphism affected both cancer incidence and longevity as well. The TP53 codon 72 polymorphism has previously been shown to influence the apoptotic potential of human cells in response to oxidative stress. Here, we studied the influence of this polymorphism on the cellular responses to X-irradiation of fibroblasts obtained from nonagenarians. We found that the average clonogenic survival after X-irradiation was similar for the three TP53 codon 72 genotype groups. As described before, X-irradiation did not induce an appreciable degree of apoptosis in human fibroblasts. However, percentages of senescence-associated (SA)-beta-galactosidase positive cells (p < 0.001), micronucleated cells (p < 0.001) and cells displaying abnormal nuclear morphologies (p < 0.001) significantly increased with the radiation dose. Compared to Arg/Arg fibroblasts, Pro/Pro fibroblasts exhibited higher irradiation dose-dependent increases in SA-beta-galactosidase positive cells (p(interaction) = 0.018), micronucleated cells (p(interaction) = 0.005) and cells displaying abnormal nuclear morphologies (p(interaction) = 0.029) at 3 days after irradiation. Possibly, these differences in cellular responses to stress between the TP53 codon 72 genotypes contribute to the differences in cancer incidence and longevity observed earlier for these genotypes.

Correlation between initial chromatid damage and survival of various cell lines exposed to heavy charged particles

The biophysical characteristics of heavy ions make them a rational source of radiation for use in radiotherapy of malignant tumours. Prior to radiotherapy treatment, a therapeutic regimen must be precisely defined, and during this stage information on individual patient radiosensitivity would be of very great medical value. There are various methods to predict radiosensitivity, but some shortfalls are difficult to avoid. The present study investigated the induction of chromatid breaks in five different cell lines, including one normal liver cell line (L02), exposed to carbon ions accelerated by the heavy ion research facility in Lanzhou (HIRFL), using chemically induced premature chromosome condensation (PCC). Previous studies have reported the number of chromatid breaks to be linearly related to the radiation dose, but the relationship between cell survival and chromatid breaks is not clear. The major result of the present study is that cellular radiosensitivity, as measured by D0, is linearly correlated with the frequency of chromatid breaks per Gy in these five cell lines. We propose that PCC may be applied to predict radiosensitivity of tumour cells exposed to heavy ions.

Prediction of cellular radiosensitivity from DNA damage induced by gamma-rays and carbon ion irradiation in canine tumor cells

Diseases of companion animals are shifting from infectious diseases to neoplasms (cancer), and since radiation therapy is one of the effective choices available for cancer treatment, the application of radiotherapy in veterinary medicine is likely to increase. However tumor tissues have different radiosensitivities, and therefore it is important to determine the intrinsic radiosensitivity of tumors in individual patients in advance of radiotherapy. We have studied the relationship between the surviving cell fraction measured by a clonogenic assay and DNA double strand breaks detected by a comet assay under neutral conditions in three canine tumor cell lines, after gamma-ray and carbon ion irradiation. In all the cell lines, cell death assessed by the clonogenic assay was much higher following irradiation with carbon ions than with gamma-rays. The initial and residual (4 hr) DNA damage due to gamma-ray and carbon ion irradiation were higher in a radiosensitive cell line than in a radioresistant cell line. The surviving cell fraction at 2 Gy (SF2) showed a tendency for correlation with both the initial and residual DNA damage. In particular, the residual damage per Gy was significantly correlated with SF2, regardless of the type of radiation. This indicates that cellular radiosensitivity can be predicted by detection of radiation-induced residual DNA damage.

Low-Dose Hyper-radiosensitivity is not Caused by a Failure to Recognize DNA Double-Strand Breaks

One of the earliest cellular responses to radiation-induced DNA damage is the phosphorylation of the histone variant H2AX (gamma-H2AX). gamma-H2AX facilitates the local concentration and focus formation of numerous repair-related proteins within the vicinity of DNA DSBs. Previously, we have shown that low-dose hyper-radiosensitivity (HRS), the excessive sensitivity of mammalian cells to very low doses of ionizing radiation, is a response specific to G(2)-phase cells and is attributed to evasion of an ATM-dependent G(2)-phase cell cycle checkpoint. To further define the mechanism of low-dose hyper-radiosensitivity, we investigated the relationship between the recognition of radiation-induced DNA double-strand breaks as defined by gamma-H2AX staining and the incidence of HRS in three pairs of isogenic cell lines with known differences in radiosensitivity and DNA repair functionality (disparate RAS, ATM or DNA-PKcs status). Marked differences between the six cell lines in cell survival were observed after high-dose exposures (>1 Gy) reflective of the DNA repair capabilities of the individual six cell lines. In contrast, the absence of functional ATM or DNA-PK activity did not affect cell survival outcome below 0.2 Gy, supporting the concept that HRS is a measure of radiation sensitivity in the absence of fully functional repair. No relationship was evident between the initial numbers of DNA DSBs scored immediately after either low- or high-dose radiation exposure with cell survival for any of the cell lines, indicating that the prevalence of HRS is not related to recognition of DNA DSBs. However, residual DNA DSB damage as indicated by the persistence of gamma-H2AX foci 4 h after exposure was significantly correlated with cell survival after exposure to 2 Gy. This observation suggests that the persistence of gamma-H2AX foci could be adopted as a surrogate assay of cellular radiosensitivity to predict clinical radiation responsiveness.

Accumulation of the common mitochondrial DNA deletion induced by ionizing radiation

Point mutations and deletions in mitochondrial DNA (mtDNA) accumulate as a result of oxidative stress, including ionizing radiation. As a result, dysfunctional mitochondria suffer from a decline in oxidative phosphorylation and increased release of superoxides and other reactive oxygen species (ROS). Through this mechanism, mitochondria have been implicated in a host of degenerative diseases. Associated with this type of damage, and serving as a marker of total mtDNA mutations and deletions, the accumulation of a specific 4977-bp deletion, known as the common deletion (Delta-mtDNA(4977)), takes place. The Delta-mtDNA(4977) has been reported to increase with age and during the progression of mitochondrial degeneration. The purpose of this study was to investigate whether ionizing radiation induces the formation of the common deletion in a variety of human cell lines and to determine if it is associated with cellular radiosensitivity. Cell lines used included eight normal human skin fibroblast lines, a radiosensitive non-transformed and an SV40 transformed ataxia telangiectasia (AT) homozygous fibroblast line, a Kearns Sayre Syndrome (KSS) line known to contain mitochondrial deletions, and five human tumor lines. The Delta-mtDNA(4977) was assessed by polymerase chain reaction (PCR). Significant levels of Delta-mtDNA(4977) accumulated 72 h after irradiation doses of 2, 5, 10 or 20 Gy in all of the normal lines with lower response in tumor cell lines, but the absolute amounts of the induced deletion were variable. There was no consistent dose-response relationship. SV40 transformed and non-transformed AT cell lines both showed significant induction of the deletion. However, the five tumor cell lines showed only a modest induction of the deletion, including the one line that was deficient in DNA damage repair. No relationship was found between sensitivity to radiation-induced deletions and sensitivity to cell killing by radiation.

Molecular mechanisms of individual radiosensitivity studied in normal diploid human fibroblasts

The molecular mechanisms of individual radiosensitivity were studied in normal diploid human fibroblasts. For fibroblasts irradiated with X-rays in G1-phase the individual radiosensitivity was shown to be correlated with the extent of double-strand break (dsb) repair. The number of residual dsbs (including both non- and mis-rejoined dsbs) varied between 2 and 5% of the initial number induced and was low for resistant and high for sensitive strains. In the G1-phase dsbs are considered to be mostly repaired via the non-homologous end-joining pathway (NHEJ). However, so far none of the parameters tested for this pathway was found to be correlated with the number of residual dsbs. The parameters tested were mRNA expression, protein level and localisation and activity of the DNA-PK, which is the central complex of NHEJ. The dsb-repair capacity is also not regulated by the differentiation status, which varies substantially among fibroblast strains, whereas there is some indication that dsb repair might depend on the chromatin structure, with more efficient repair in cells with condensed DNA. Residual dsbs are converted into lethal chromosome aberrations finally leading to the loss of clonogenic activity, when cells pass through mitosis. Beside this so-called mitotic death, X-irradiated human fibroblasts are also inactivated via the TP53-dependent permanent G1-arrest, while apoptosis appears to be not important. On average, mitotic death and G1-arrest are equally effective, but there is a broad variation from one strain to the other, with a negative correlation between these two pathways. Fibroblast strains exhibiting only a moderate G1-arrest showed a high number of lethal aberrations and vice versa. This result points to a common regulator of both G1-arrest and dsb repair, which is presently under investigation.

Radiosensitivity of human tumour cells is correlated with the induction but not with the repair of DNA double-strand breaks

Nine human tumour cell lines (four mammary, one bladder, two prostate, one cervical, and one squamous cell carcinoma) were studied as to whether cellular radiosensitivity is related to the number of initial or residual double-strand breaks (dsb). Cellular sensitivity was measured by colony assay and dsb by means of constant- and graded-field gel electrophoresis (CFGE and GFGE, respectively). The nine tumour cell lines showed a broad variation in cellular sensitivity (SF2 0.17-0.63). The number of initial dsb as measured by GFGE ranged between 14 and 27 dsb/Gy/diploid DNA content. In contrast, normal fibroblasts raised from skin biopsies of seven individuals showed only a marginal variation with 18-20 dsb/Gy/diploid DNA content. For eight of the nine tumour cell lines, there was a significant correlation between the number of initial dsb and the cellular radiosensitivity. The tumour cells showed a broad variation in the amount of dsb measured 24 h after irradiation by CFGE, which, however, was not correlated with the cellular sensitivity. This residual damage was found to be influenced not only by the actual number of residual dsb, but also by apoptosis and cell cycle progression which had impact on CFGE measurements. Some cell line strains were able to proliferate even after exposure to 150 Gy while others were found to degrade their DNA. Our results suggest that for tumour cells, in contrast to normal cells, the variation in sensitivity is mainly determined by differences in the initial number of dsb induced.

The relationship between cellular radiosensitivity and radiation-induced DNA damage measured by the comet assay

The relationship between deoxyribonucleic acid (DNA) damage and the cell death induced by gamma-irradiation was examined in three kinds of cells, Chinese hamster ovary fibroblast CHO-K1, human melanoma HMV-II and mouse leukemia L5178Y. Cell survival was determined by a clonogenic assay. The induction and rejoining of DNA strand breaks induced by radiation were measured by the alkaline and neutral comet assays. L5178Y cells were the most radiosensitive, while CHO-K1 cells and HMV-II cells were radioresistant. There was an inverse relationship between the survival fraction at 2 Gy (SF2) and the yield of initial DNA strand breaks per unit dose under the alkaline condition for the comet assay, and also a relationship between SF2 and the residual DNA strand breaks (for 4 hr after irradiation) under the neutral condition for the comet assay, the latter being generally considered to be relative to cellular radiosensitivity. In the present analysis, it was considered that the alkaline condition for the comet assay was optimal for evaluating the initial DNA strand breaks, while the neutral condition was optimal for evaluating the residual DNA strand breaks. Since the comet assay is simpler and more rapid than other methods for detecting radiation-induced DNA damage, this assay appears to be a useful predictive assay for evaluating cellular clonogenic radiosensitivity of tumor cells.

Does skin fibroblast radiosensitivity predict squamous cancer cell radiosensitivity of the same individual?

Individualization of radiation doses is presumed to result in better radiotherapy outcome. Success rate in measuring radiosensitivity is probably the most limiting factor for present radiosensitivity assays to be introduced into clinical routine. To find a simpler predictive parameter, we compared the radiosensitivity of dermal fibroblasts and head and neck squamous cell carcinoma (SCC) cell lines established from the same individuals. The radiosensitivity was tested using the clonogenic 96-well plate assay. The surviving fraction at 2.0 Gy (SF2) was determined, as well as the mean inactivation dose (AUC) of cancer cells. SF2 of SCC cell lines and skin fibroblasts were 0.25-0.44 and 0.11-0.43, respectively. AUC of SCC cells was 1.4-2.1 Gy. Dermal fibroblasts were more radiosensitive than SCC cells in 14 of 15 cases. In 1 patient (UT-SCC-8), cancer cells were found to be more radiosensitive than corresponding dermal fibroblasts. There was a clear tendency to a correlation between radiosensitivities of these 2 cell types, but statistical significance was reached only when the data of UT-SCC-8 was excluded. In our material, the intrinsic radiosensitivity of head and neck SCC cells could in most cases be predicted from the intrinsic radiosensitivity of dermal fibroblasts established from the same individual.

Relationship between residual radiation-induced DNA double-strand breaks in cultured fibroblasts and late radiation reactions: a comparison of training and validation cohorts of breast cancer patients

BACKGROUND AND PURPOSE

Using pulsed field gel electrophoresis (PFGE) a significant correlation was demonstrated between residual DNA double-strand breaks (dsbs) and the development of late radiation fibrosis in a group of 39 breast cancer patients studied retrospectively. This group formed a training cohort generating a hypothesis that there is a relationship between residual radiation-induced DNA dsbs in cultured fibroblasts and late radiotherapy reactions in breast cancer patients. The aim of this study was to retest and validate the hypothesis.

MATERIALS AND METHODS

The study was retrospective. Skin biopsies were taken from a validation cohort of 50 breast cancer patients and PFGE was used to examine residual radiation-induced dsbs in cultured fibroblasts. Late morbidity was measured clinically as fibrosis and using the late effects on normal tissues scales that incorporate subjective, objective management and analytic data (LENT SOMA).

RESULTS

PFGE data were obtained for 49 biopsies. In the 49 patients there was no correlation between residual DNA damage and either fibrosis (r=-0.027, P=0.85) or LENT SOMA (r=-0.10, P=0.48) scores. There was no significant relationship between residual damage and fibrosis for the combined training and validation cohorts of 88 patients (r=0.20, P=0.063).

CONCLUSIONS

This study did not validate the hypothesis that there is a relationship between fibroblast residual DNA damage and late morbidity in breast cancer patients. The PFGE assay on fibroblasts is not a suitable test of the degree of late radiation-induced fibrosis in the breast.

DNA double-strand breaks trigger apoptosis in p53-deficient fibroblasts

DNA double-strand breaks (DSBs) are induced by ionizing radiation (IR) and various radiomimetic agents directly, or indirectly as a consequence of DNA repair, recombination and replication of damaged DNA. They are ultimately involved in the generation of chromosomal aberrations and were reported to cause genomic instability, gene amplification and reproductive cell death. To address the question of whether DSBs act as a trigger of apoptosis, we induced DSBs by means of restriction enzyme electroporation and compared the effect with IR in mouse fibroblasts that differ in p53 status [wild-type (+/+) versus p53-deficient (-/-) cells]. We show that (i) electroporation of PVU:II is highly efficient in the induction of DSBs, (ii) electroporation of PVU:II increases the rate of apoptosis, but not of necrosis in p53-/- cells, (iii) treatment with gamma-rays induces both apoptosis and necrosis in p53-/- cells, (iv) the frequency of DSBs correlates with the yield of apoptosis and (v) both PVU:II and gamma-ray treatment reduce the level of anti-apoptotic Bcl-2 protein in p53-/- cells whereas the level of Bax remains unaltered. Cells expressing wild-type p53 were more resistant than p53-deficient cells as to the induction of apoptosis and did not show Bcl-2 decline upon treatment with PVU:II and gamma-rays. The data provide evidence that blunt-ended DSBs induced by restriction enzyme PVU:II act as a highly efficient trigger of apoptosis, but not of necrosis. This process is related to Bcl-2 decline and does not require p53.

Normal tissue radiobiology: from the laboratory to the clinic

This manuscript is in four parts, presenting the four talks given in a symposium on normal tissue radiobiology. The first part addresses the general concept of the role of parenchymal cell radiosensitivity vs. other factors, highlighting research over the last decade that has altered our understanding of factors underlying normal tissue response. The other three parts expand on specific themes raised in the first part dealing in particular with (1) modifications of fibroblast response to irradiation in relation to the induction of tissue fibrosis, (2) the use of the linear-quadratic equation to model the potential benefits of using different means (both physical and biologic) of modifying normal tissue response, and (3) the specific role of the growth factor TFG-beta1 in normal tissue response to irradiation. The symposium highlights the complexities of the radiobiology of late normal tissue responses, yet provides evidence and ideas about how the clinical problem of such responses may be modified or alleviated.

Relative biological effectiveness for cell-killing effect on various human cell lines irradiated with heavy-ion medical accelerator in Chiba (HIMAC) carbon-ion beams

PURPOSE

To clarify the relative biological effectiveness (RBE) values of various human cell lines for carbon-ion beams with 2 different linear energy transfer (LET) beams and to investigate the relationship between the cell-killing effect and the biophysical characters, such as the chromosome number and the area of the cell nucleus, using qualitatively different kinds of radiations.

METHODS AND MATERIALS

Sixteen different human cell lines were irradiated with carbon-ion beams, having 2 different LET values (LET(infinity) = 13.3 and approximately 77 keV/microm), accelerated by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences in Japan. Cell-killing effect was detected as reproductive cell death using a colony-formation assay. The number of chromosomes was observed in a metaphase spread using the conventional method. The area of the cell nucleus was calculated as an ellipse on photographs using a micrometer.

RESULTS

The RBE values calculated by the D(10), which is determined as the dose (Gy) required to reduce the surviving fraction to 10%, relative to X-rays, range from 1.06 to 1.33 for 13-keV/microm-beam and from 2.00 to 3. 01 for approximate 77-keV/microm-beam irradiation on each cell line. There was a good correlation in the D(10) values of each cell line between X-rays and carbon-ion beams. However, the D(10) values did not clearly depend on either the chromosome number or the area of the cell nuclei.

CONCLUSION

The RBE values for HIMAC carbon-ion beams are consistent with previous reports using carbon-ion beams with the similar LET values, and the cellular radiosensitivity of different cell lines well correlate among different types of radiation.

Relationship between DNA double-strand breaks, cell killing, and fibrosis studied in confluent skin fibroblasts derived from breast cancer patients

PURPOSE

To investigate the relationship between DNA double-strand breaks (dsbs), cell killing, and fibrosis using skin fibroblasts derived from breast cancer patients who received postmastectomy radiotherapy.

METHODS AND MATERIALS

Experiments were performed with 12 lines of normal skin fibroblasts derived from recurrence-free breast cancer patients. Cells were irradiated in confluence and cell survival was determined either after immediate or delayed (14 h) plating using a colony-forming assay. Dsbs were measured by constant-field gel electrophoresis. The "excess risk of fibrosis" was previously scored by Johansen et al. (IJRB 1994;66:407-412).

RESULTS

The 12 cell lines showed a typical spectrum of radiosensitivity. The mean value of surviving fraction after 3.5 Gy (SF3.5) was 0.063 for immediate and 0.174 for delayed plating with a coefficient of variation (CV) of 44 and 39%, respectively. There was also a broad variation in the extent of recovery from potentially lethal damage (RPLD), which was not correlated with the immediate sensitivity. The number of initial dsbs as well as the half-times of dsb repair showed little variation, whereas there were considerable differences in the number of residual dsbs (CV = 29%). The number of residual dsbs after 100 Gy was correlated significantly only with SF3.5 after delayed (r2 = O.59; p = 0.006) but not after immediate plating (r2 = 0.21, p = 0.16). There was also no significant relationship between residual dsbs and the "excess risk of fibrosis" determined for the respective patients.

CONCLUSION

It is shown that the number of residual dsbs measured in confluent human fibroblast lines can be used to predict the cellular radiosensitivity after delayed but not after immediate plating and also not to predict the excess risk of fibrosis of the respective breast cancer patients.

A correlation between residual radiation-induced DNA double-strand breaks in cultured fibroblasts and late radiotherapy reactions in breast cancer patients

BACKGROUND AND PURPOSE

Prediction of late normal tissue reactions to radiotherapy would permit tailoring of dosage to each patient. Measurement of residual DNA double strand breaks using pulsed field gel electrophoresis (PFGE) shows promise in this field. The aim of this study was to test the predictive potential of PFGE in a group of retrospectively studied breast cancer patients.

MATERIALS AND METHODS

Thirty nine patients, treated uniformly for breast cancer 9-15 years previously, with excision of the tumour and radiotherapy to the breast and drainage areas, were assessed clinically using the LENT SOMA scale, and a 5-mm punch biopsy taken from the buttock. Fibroblast cell strains were established and used to study residual DNA double strand breaks, using PFGE.

RESULTS

There were significant correlations between the DNA assay results and the fibrosis score (r(s) = 0.46; P = 0.003), the combined fibrosis and retraction score (r(s) = 0.45, P = 0.004) and the overall LENT score (r(s) = 0.43; P = 0.006). Using polychotomous logistic regression, the fibroblast DNA assay result was an independent prognostic factor for fibrosis severity.

CONCLUSIONS

There is a relationship between residual radiation-induced DNA damage in fibroblasts and the severity of the late normal tissue damage seen in the patients from whom the cells were cultured.

Fibroblast radiosensitivity measured using the comet DNA-damage assay correlates with clonogenic survival parameters

A study was made of the neutral comet assay as a potential method for measuring normal cell radiosensitivity. Eleven fibroblast strains were studied comprising nine derived from vaginal biopsies from pretreatment cervical cancer patients and two strains from radiosensitive individuals. DNA double strand break (dsbs) dose-response curves for both initial and residual (20-h repair time) damage were obtained over the dose range 0-240 Gy, with slopes varying 3.2 and 8-fold respectively. Clonogenic cell survival parameters were available for all the cell strains following both high- and low-dose rate irradiation. There were no correlations between the dose-response slope of the initial level of DNA dsbs and parameters that mainly describe the initial portion of clonogenic radiation survival curves (SF2, alpha, D). A significant correlation (r = -0.63, P = 0.04) was found between the extent of residual DNA dsbs and clonogenicity for all 11 fibroblast strains. The parameter showing the highest correlation with fibroblast cell killing (D) for the nine normal fibroblasts alone was the ratio of initial/residual DNA dsb dose-response slope (r = 0.80, P = < 0.01). A significant correlation (r = -0.67, P = 0.03) with clonogenic radiosensitivity was also found for all 11 cell strains when using the ratio of initial/residual DNA dsb damage at a single dose of 180 Gy. This study shows that fibroblast radiosensitivity measured using the neutral comet assay correlates with clonogenic radiation survival parameters, and therefore may have potential value in predictive testing of normal tissue radiosensitivity.

Lack of correlation between residual radiation-induced DNA damage, in keratinocytes assayed directly from skin, and late radiotherapy reactions in breast cancer patients

PURPOSE

To study the relationship between the severity of late reactions to radiotherapy in breast cancer patients, and the extent of residual radiation-induced DNA damage, using a rapid assay of keratinocytes obtained directly from skin biopsies.

METHODS AND MATERIALS

A review was made of 32 patients with breast cancer, treated uniformly by radiotherapy between 1983 and 1988, following breast-conserving surgery. Their late radiotherapy reactions were scored (9-14 years post-radiotherapy) using a modified LENT SOMA scale, and a 5-mm buttock skin punch biopsy was obtained. Intact skin was irradiated at room temperature, and after allowing 24 h for repair, the tissue was disaggregated and the cells processed for pulsed field gel electrophoresis (PFGE). Residual DNA damage was expressed as the fraction of DNA released (FDR) following 150 Gy.

RESULTS

Studies using flow cytometry on disaggregated breast skin showed that over 90% of the cells were keratinocytes. The PFGE assay was robust with low background FDRs in unirradiated skin samples (mean 3.2%) and a wide range of FDRs following irradiation from 11.5% to 26.6%. No correlation was found between the FDR at 150 Gy (FDR 150) and any of the late reaction scores or retrospective acute reaction scores. There was, however, a borderline significant correlation for family history and FDR 150 (p = 0.059).

CONCLUSION

Rapid measurement of residual DNA damage in irradiated differentiated keratinocytes, the predominant cell population in skin biopsies, showed no correlation with the severity of symptomatic early or documented late reactions in a retrospectively studied group of 32 breast cancer patients.

DNA-dependent protein kinase content and activity in lung carcinoma cell lines: correlation with intrinsic radiosensitivity

Intrinsic radiosensitivity and rejoining of radiation-induced DNA double-strand breaks (DNA-dsb) were analysed in five lung carcinoma cell lines: U-1285, U-1906, H-69, H-82 and U-1810. RS correlated with both the initial phase of DNA-dsb rejoining, at 15 min (r2 = 0.818) and the late phase, at 120 min postirradiation (r2 = 0.774), the most sensitive cell line (U-1285) showing least dsb rejoining and the most resistant (U-1810) showing most dsb rejoining of all five cell lines studied. As DNA-PK has been recognised as an important molecular component involved in DNA-dsb repair, we analysed content and activity of this kinase. We found that DNA-PK content and activity correlated with RS (r2 = 0.941 and r2 = 0.944, respectively). The lowest DNA-dependent content/activity was found in the most radiosensitive cells, U-1285 and H-69, whilst the highest content/activity was found in the most radioresistant cells U-1810. These results suggest a correlation between RS and DNA-PK content/activity in lung carcinoma cell lines.

Levels of the DNA repair enzyme human apurinic/apyrimidinic endonuclease (APE1, APEX, Ref-1) are associated with the intrinsic radiosensitivity of cervical cancers

A study was made of the relationship between the intrinsic radiosensitivity of human cervical tumours and the expression of the DNA repair enzyme human apurinic/apyrimidinic endonuclease (HAP1). The radiosensitivity of clonogenic cells in tumour biopsies was measured as surviving fraction at 2 Gy (SF2) using a soft agar assay. HAP1 expression levels were determined after staining of formalin-fixed paraffin-embedded tumour sections with a rabbit antiserum raised against recombinant HAP1. Both measurements were obtained on pretreatment biopsy material. All 25 tumours examined showed positive staining for HAP1, but there was heterogeneity in the level of expression both within and between tumours. The average coefficients of variation for intra- and intertumour heterogeneity were 62% and 82% respectively. There was a moderate but significant positive correlation between the levels of HAP1 expression and SF2 (r = 0.60, P = 0.002). Hence, this study shows that there is some relationship between intrinsic radiosensitivity and expression of a DNA repair enzyme in cervical carcinomas. The results suggest that this type of approach may be useful in the development of rapid predictive tests of tumour radiosensitivity.

Differentiation state of skin fibroblast cultures versus risk of subcutaneous fibrosis after radiotherapy

BACKGROUND AND PURPOSE

There is increasing evidence for patient-to-patient variation in the response of normal tissue to radiotherapy. Recently, it has been suggested that accumulation of functional fibrocytes may be a key step in the development of radiation-induced fibrosis. Therefore, we have examined a possible relationship between the differentiation state of untreated fibroblasts and the risk of radiation-induced subcutaneous fibrosis in individual patients.

MATERIALS AND METHODS

We used skin fibroblast cultures isolated from eight postmastectomy radiotherapy patients whose individual clinical radiosensitivity was assessed by the mean excess risk of fibrosis. Different types of potentially mitotic progenitor fibroblasts (MF) and postmitotic functional fibrocytes (PMF) in the terminal differentiation lineage, MFI --> MFII --> MFIII --> PMF, were scored morphologically in clonal culture. Progression of differentiation was quantified by the ratio L/E of colony-forming late (MFIII and late MFII) and early (MFI and early MFII) progenitors.

RESULTS

We observed a correlation between the ratio L/E and the mean risk of fibrosis (rs = 0.743, P = 0.03), indicating an approximately 10-fold increase in L/E with an increasing risk of fibrosis. This was paralleled by a decreasing trend in the absolute numbers of early progenitor types. By contrast, there was no significant correlation between the plating efficiency and the risk of fibrosis.

CONCLUSIONS

The data suggest that the risk of fibrosis increases with the progression of the differentiation of untreated progenitor fibroblasts, indicating that the progression of fibroblast differentiation may be a co-factor in the development of radiation-induced fibrosis. If this hypothesis is validated, it provides a rationale for a novel predictive test to identify patients with an increased risk of subcutaneous fibrosis.

The radiosensitivity of human fibroblast cell lines correlates with residual levels of DNA double-strand breaks

PURPOSE

To study the correlation of residual DNA double-strand breakage after irradiation and cellular radiosensitivity in cells showing marked differences in radiosensitivity.

MATERIALS AND METHODS

The levels of DNA double-strand breaks remaining at 4 h after irradiation were measured by graded-voltage gel electrophoresis in fibroblast cell strains derived from seven individuals either with normal radiosensitivity (n = 2), or with genetic abnormalities known to show increased (two ataxia telangiectasia, one scid) or possibly decreased (two Li-Fraumeni family members) sensitivity.

RESULTS

The slope of the dose-response curve for DNA breaks remaining unrepaired at 4 h showed a highly significant correlation with cellular radiosensitivity characterized by SF2, alpha, or D (r > or = 0.91, P < 0.001). Hence, this measure of genotoxic damage was predictive of radiation sensitivity for cells affected by a variety of mutations in different damage signalling/repair components.

DISCUSSION

This correlation confirms another published study and extends it to cell lines with other genetic defects. The technique may be useful in the development of rapid assays to predict the sensitivity of normal tissues in patients receiving radiotherapy.

The ratio of initial/residual DNA damage predicts intrinsic radiosensitivity in seven cervix carcinoma cell lines

The single-cell gel electrophoresis (comet) assay was used to measure radiation-produced DNA double-strand breaks (dsbs) in a series of seven cervical tumour cell lines (ME180, HT3, C33A, C41, SiHa, MS751 and CaSki). The proportion of DNA dsbs was measured immediately after radiation treatment (initial damage) and 16 h later after incubation at 37 degrees C (residual damage). Linear dose-response curves were seen for initial (slopes 0.23-0.66) and residual (slopes 0.16-0.87) DNA dsbs. Neither of the slopes of the linear regression analysis on the initial and on the residual DNA dsbs dose-response curves (range 0-80 Gy) correlated with SF2 (surviving fraction at 2 Gy) measured after high- (HDR) or low-dose-rate (LDR) irradiation. An association was evident between SF2 after HDR and LDR irradiation and the ratio of the absolute level of initial and residual damage after a single dose of 60 Gy. However, a significant correlation was found between HDR (r= -0.78, P = 0.04) and LDR (r = -0.86, P = 0.03) SF2 values and the ratio of the slopes of the initial and residual DNA dsbs dose-response curves (range 0.47-0.99), representing the fraction of DNA damage remaining. These results indicate that the neutral comet assay can be used to predict radiosensitivity of cervical tumour cell lines by assessing the ratio of initial and residual DNA dsbs.

Late G1 accumulation after 2 Gy of gamma-irradiation is related to endogenous Raf-1 protein expression and intrinsic radiosensitivity in human cells

We have previously reported a correlation between high endogenous expression of the protein product of the RAF-1 proto-oncogene, intrinsic cellular radiosensitivity and rapid exit from a G2/M delay induced by 2 Gy of gamma-irradiation. Raf1 is a positive serine/threonine kinase signal transduction factor that relays signals from the cell membrane to the MAP kinase system further downstream and is believed to be involved in an ionizing radiation signal transduction pathway modulating the G1/S checkpoint. We therefore extended our flow cytometric studies to investigate relationships between radiosensitivity, endogenous expression of the Raf1 protein and perturbation of cell cycle checkpoints, leading to alterations in the G1, S and G2/M populations after 2 Gy of gamma-irradiation. Differences in intrinsic radiosensitivity after modulation of the G1/S checkpoint have generally been understood to involve p53 function up to the present time. A role for dominant oncogenes in control of G1/S transit in radiation-treated cells has not been identified previously. Here, we show in 12 human in vitro cancer cell lines that late G1 accumulation after 2 Gy of radiation is related to both Raf1 expression (r = 0.91, P = 0.0001) and the radiosensitivity parameter SF2 (r = -0.71, P = 0.009).

[Radiation-induced DNA fragmentation evaluated by anti-poly(ADP-ribose)immunolabeling in CHO cells. Standardization with pulsed-field electrophoresis]

The poly (ADP-ribose) polymerase is an ubiquitous nuclear protein capable of binding specifically to DNA strand breaks. It synthesizes ADP-ribose polymers proportionally to DNA breaks. The actual method of reference to determine DNA double strand breaks is pulsed-field gel electrophoresis, but this requires many cells. It thus appeared of interest to use poly (ADP-ribos)ylation to follow and estimate gamma-ray-induced DNA fragmentation at the level of isolated cells after gamma-irradiation in chinese hamster ovary cells (CHO-K1). The results obtained by the immunolabelling technique of ADP-ribose polymers were compared to those obtained by pulsed-field gel electrophoresis. They show that poly (ADP-ribos)ylation reflects the occurrence of radiation-induced DNA strand breaks. A clear relationship exists between the amount of ADP-ribose polymers detected and DNA double strand breaks after gamma-irradiation.

A correlation between residual DNA double-strand breaks and clonogenic measurements of radiosensitivity in fibroblasts from preradiotherapy cervix cancer patients

PURPOSE

To study the relationship between residual DNA damage and clonogenic measurements of radiosensitivity in fibroblasts from pretreatment cervix cancer patients.

METHODS AND MATERIALS

Early passage vaginal fibroblasts from nine preradiotherapy cervix cancer patients and two radiosensitive skin fibroblast cell strains were studied. Cell survival was measured by clonogenic assay following both high and low dose rate irradiation. Residual DNA damage was measured using pulsed-field gel electrophoresis (PFGE) after irradiating radiolabeled, plateau-phase cells at 37 degrees C and allowing 24 h for repair. DNA damage was expressed both in terms of the residual damage slope (fitted to data from 60 to 150 Gy) and the fraction of activity released (FAR) following 150 Gy.

RESULTS

The surviving fraction at 2 Gy (SF2) values after high dose rate irradiation for the vaginal fibroblasts ranged from 0.15 to 0.32 (a 2.2-fold difference). When the two radiosensitive cell strains were included, residual damage, expressed as the residual damage slope, correlated with alpha (r = 0.82, p = 0.002), D bar (r = -0.91, p < 0.001) and SF2 (p = -0.79, p = 0.004), and when the vaginal fibroblasts alone were studied, the residual damage slope again correlated with clonogenic survival, although less strongly [alpha (r = 0.66, p = 0.053), D bar (r = -0.83, p = 0.006), and SF2 (r = -0.63, p = 0.07)]. Within the group of vaginal fibroblasts there was a 4.0-fold difference in residual DNA damage slope. When residual damage was expressed as FAR at 150 Gy, then for all cell strains the correlations were alpha: r = 0.78, p = 0.004, D bar: r = -0.86, p = 0.001, and SF2: r = -0.78, p = 0.004, and for the vaginal fibroblast strains alone the correlations were alpha: r = 0.60, p = 0.088, D bar: r = -0.75, p = 0.02, and SF2: r = 0.62, p = 0.077.

CONCLUSION

This study confirms previous findings that residual DNA damage correlates with clonogenic survival in fibroblasts. In addition, it demonstrates a correlation for fibroblasts from pretreatment cervix cancer patients demonstrating a relatively small range of SF2 values.

Radiation-induced DNA double-strand breaks and the radiosensitivity of human cells: a closer look

A large number of reports suggest that DNA double-strand breaks (DSB) play a major role in the radiation-induced killing of mammalian cells. However, the arguments supporting the relationship between DSB and radiosensitivity are generally indirect. Furthermore, care must be taken to allow for the possible impact of the techniques and of the experimental protocols on the relationship between DSB and cell death. The recent data on DSB induction, repair and misrepair in human cell lines and their correlation with intrinsic radiosensitivity are reviewed.

Hypersensitivity of ataxia telangiectasia fibroblasts to ionizing radiation is associated with a repair deficiency of DNA double-strand breaks

We have studied the intrinsic radiosensitivity, repair of potentially lethal damage (PLD) and the repair rate of radiation-induced DNA double-strand breaks (DSB) in 11 non-transformed human fibroblast cell lines, four of which were homozygous for the A-T mutation and two that were heterozygous (A-TH). All the experiments were done on cells in plateau phase of growth (97-99% of cells in G0/G1). With a dose of 30 Gy delivered at 4 degrees C, the A-T cell lines had faster repair rates of up to 6 h, after which the repair curve crossed that of the control so that the residual damage at 24 h was higher in the A-T cells. Irradiation at 37 degrees C at low dose rate 1 cGy.min-1) produced even more marked differences between the A-T cells and controls: the residual DSB level was always higher in A-T cells than controls at doses of 5-40 Gy, due to defective repair of a small fraction of DSB in A-T cells. The two protocols showed DSB repair rates for the A-TH cell lines that were intermediate between those of the A-T and control cells. There was a quantitative relationship between the residual DSB after irradiation at 37 degrees C and the intrinsic radiosensitivity, and with the extent of PLD repair. There were very few apoptotic cells in the non-transformed control and A-T cell line, both before and after irradiation. In combination, these result support the contention that the defective repair of DSB is a mechanism of the hypersensitivity linked to the A-T mutation.

A role for molecular radiobiology in radiotherapy?

As we learn more about the cellular response to radiation and its genetic control, new avenues are opened up that have the potential to have a significant impact on radiotherapy practice. The recognition of the importance of the control of DNA damage induction and repair, cell cycle arrest and apoptosis gives us the primary areas to investigate, and the improvements in molecular technology make the application of our new knowledge more feasible. It can only be hoped that specific means can be found to assist in the prediction of normal tissue and tumour radiosensitivity and to manipulate sensitivity when that is desirable.

Comparative measurements of radiation-induced DNA double-strand breaks by graded voltage and pulsed-field gel electrophoresis

We compared the ability of graded-voltage gel electrophoresis (GVGE) and pulsed-field gel electrophoresis (PFGE), to resolve DNA containing double-strand breaks (dsb) induced in human MS751 cells after exposure to gamma-radiation. For quantitation, prelabelling of the cells with [2-14C]-thymidine prior to electrophoresis and subsequent scintillation counting of excised bands was found to give closely similar results to Southern blotting and radiolabelled probe hybridization followed by phosphorimager quantitation. Compared with PFGE, DNA subjected to GVGE migrated further and generated distinct DNA bands. Dsb were detected and quantifiable with GVGE at much lower doses than with PFGE the radiation dose limits were approximately 2 and 10 Gy respectively. At all doses used, the amounts of dsb detected by GVGF, were higher than those by PFGE. GVGE coupled with Southern hybridization and phosphorimager analysis is thus a more sensitive approach to assessing dsb and their relationship with radiation sensitivity. The approach is also convenient when processing large numbers of samples and has the additional advantage of avoiding cell prelabelling such as in the case of cells extracted directly from human tumour biopsies and normal tissues.

Chromatin structure and cellular radiosensitivity: a comparison of two human tumour cell lines

The role of variation in susceptibility to DNA damage induction was studied as a determinant for cellular radiosensitivity. Comparison of the radiosensitive HX142 and radioresistant RT112 cell lines previously revealed higher susceptibility to X-ray-induced DNA damage in the sensitive cell line using non-denaturing elution, but not when using alkaline unwinding. The present data also show that no difference in the amount of initial damage is seen when pulsed-field gel electrophoresis (PFGE) or comet analysis are used for DNA damage assessment. However, using the halo assay or a modified version of PFGE in which the higher DNA architecture remained partially intact, the radiosensitive cells showed steeper dose-response curves for initial DNA damage than the radioresistant cells. Analysis of the protein composition, of DNA-nucleoid structures revealed substantial differences when isolated from HX142 or RT112 cells. From our data, it is concluded that HX142 and RT112 differ in their structural organization of chromatin. As no differences in the kinetics of DNA damage rejoining were found, it is hypothesized that the same amount of lesions have a different impact in the two cell lines in that the 'presentation' of DNA damage alters the ratio of repairable to non-repairable DNA damage.

Feasibility of measuring radiation-induced DNA double strand breaks and their repair by pulsed field gel electrophoresis in freshly isolated cells from the mouse RIF-1 tumor

PURPOSE

To examine the technical feasibility of pulsed field gel electrophoresis (PFGE) as a predictive assay for the radioresponsiveness of tumors. Induction and repair of DNA double strand breaks (DSBs) in a freshly prepared cell suspension from a RIF-1 tumor (irradiated ex vivo) was compared with DSB induction and repair in exponentially growing RIF-1 cells in culture (irradiated in vitro).

METHODS AND MATERIALS

A murine RIF-1 tumor grown in vivo was digested, and cells were exposed to x-rays (ex vivo) at doses of 1 to 75 Gy. DNA damage was measured using CHEF (clamped homogeneous electric fields) electrophoresis. Repair kinetics were studied at 37 degrees C for 4 h after irradiation. Radiosensitivity was determined by clonogenic assay, and cell cycle distributions by flow cytometry. For comparison, a trypsinized suspension of exponentially growing RIF-1 cells in vitro was run parallel with each ex vivo experiment.

RESULTS

Induction of DSBs, expressed as % DNA extracted from the plug, was similar in the in vitro and ex vivo irradiated cells. Compared to repair rates in vitro cultured RIF-1 cells, repair kinetics in a freshly prepared cell suspension from the tumor were decreased, unrelated to differences in radiosensitivity. Differences in repair could not be explained by endogenous DNA degradation, nor by influences of enzymes used for digestion of the tumor. A lower plating efficiency and differences in ploidy (as revealed by flow cytometry) were the only reproducible differences between in vivo and in vitro grown cells that may explain the differences in repair kinetics.

CONCLUSIONS

The current results do not support the idea that PFGE is a technique robust enough to be a predictive assay for the radiosensitivity of tumor cells.

Further studies on the possible relationship between radiation-induced reciprocal translocations and intrinsic radiosensitivity of human tumor cells

BACKGROUND AND PURPOSE

The aim of the present study was to estimate yields of radiation-induced translocations in surviving cells of several human tumor cell lines and in normal diploid human fibroblasts, and to compare these yields with corresponding intrinsic radiosensitivities determined by standard colony-formation assay.

MATERIAL AND METHODS

The yields of radiation-induced reciprocal translocations were investigated by fluorescence in situ hybridization. Chromosomes no. 1 and no. 4 were 'painted' with fluorescent hybridization probes for whole chromosomes. Translocation yields and cell survival were determined for different doses up to 6 Gy of 200 kV X-rays.

RESULTS

We observed a higher frequency of reciprocal translocations in the radiosensitive cells MCF-7 and MDA-MB-436 than in the radioresistant cells CaSki, WiDr, A549 and normal skin fibroblasts. For primary squamous cell carcinoma cells, ZMK-1, an intermediate radiosensitivity and an intermediate translocation yield were observed. The dose-dependence of translocation yields involving chromosomes no. 1 or no. 4 varied in different cell lines: it was linear or linear with a plateau at higher doses.

CONCLUSIONS

A comparison of the data obtained with chromosomes no. 1 and no. 4 in the investigated cell types, indicates that intrinsic radiosensitivity of different tumor cells observed at the survival level, is correlated with different translocation yields, respectively. This correlation was observed for all cell types investigated, independent of the number of copies of the painted chromosome per cell or the radiation dose. However, for low doses (under 1 Gy), the yields of translocations determined for the individual chromosomes seem to be too low for a discrimination between radioresistant or radiosensitive cells.

Relationship between DNA damage, rejoining and cell killing by radiation in mammalian cells

The prevailing hypothesis on the mechanism of radiation-induced cell killing identifies the genetic material deoxyribonucleic acid (DNA) as the most important subcellular target at biologically relevant doses. In this review we present new data and summarize the role of the DNA double-strand breaks (dsb) induced by ionizing radiation and DNA dsb rejoining as determinants of cellular radiosensitivity. When cells were irradiated at high dose-rate, two molecular end-points were identified which often correlated with radiosensitivity: (1) the apparent number of DNA dsb induced per Gy per DNA unit and (2) the half-time of the fast component of the DNA dsb rejoining kinetics. These two molecular determinants, not mutually exclusive, may be linked through a common factor such as the conformation of DNA.

Rejoining of DNA double-strand breaks in X-irradiated CHO cells studied by constant- and graded-field gel electrophoresis

Induction and repair of double-strand breaks (dsb) were measured in exponentially growing CHO-10A cells using the constant- and graded-field gel electrophoresis. Dsb repair was studied after an X-ray dose of 60 Gy. The repair curve obtained was biphasic with the respective half-times of tau 1 = 3.8 +/- 0.9 and tau 2 = 118 +/- 30 min. The number of non-reparable dsb was measured for X-ray doses up to 180 Gy and was found to be only a small fraction (14%) of all non-rejoinable breaks determined previously using the alkaline unwinding technique. The ratio of non-reparable dsb to the number of lethal events calculated from survival curves is 0.14:1. This result indicates that for CHO cells nonreparable dsb represent only a small fraction of lethal damage. This is in line with the cytogenetic observation that cell killing mainly results from mis-rejoined events (i.e. exchange aberrations, translocations, interstitial deletions). The kinetics of dsb rejoining were found to be independent of the size of the fragments involved (between 1 and 10 Mbp). In addition, the rejoining kinetics of DNA fragments < or = 1 Mbp did not show the formation of new DNA fragments with time after irradiation indicating the absence of programmed cell death in irradiated CHO cells.

Relationship between p53 status and radiosensitivity in human tumour cell lines

We examined the relationship between p53 levels before and after irradiation, radiation-induced cell cycle delays, apoptotic cell death and radiosensitivity in a panel of eight human tumour cell lines. The cell lines differed widely in their clonogenic survival after radiation, (surviving fraction at 2 Gy: SF2=0.18-0.82). Constitutive p53 protein levels varied from 2.2 +/- 0.4 to 6.3 +/- 0.3 optical density units (OD) per 10(6) cells. p53 after irradiation (6 Gy) also varied between the cell lines, ranging from no induction to a 1.6-fold increase in p53 levels 4 h after treatment. p53 function was also assessed by G1 cell cycle arrest after irradiation. The cellular response to radiation, measured as G0/G1 arrest, and the induction of apoptosis were in good agreement. However, a trace amount of DNA ladder formation was found in two cell lines lacking G1 arrest. Overall cellular radiosensitivity correlated well with the level of radiation-induced G1 arrest (correlation coefficient r=0.856; P=0.0067), with p53 constitutive levels (r=0.874, P=0.0046), and with p53 protein fold induction (r=-0.882, P=0.0038). Our data suggest that (1) the constitutive p53 level, (2) G1 arrest after irradiation, or (3) the p53 protein response to radiation may be good predictive tests for radiosensitivity in some cell types.

Dose-rate effect on radiation-induced DNA double-strand breaks in the human fibroblast HF19 cell line

We measured DNA double-strand breaks (dsbs) immediately after exposure of a non-transformed human fibroblast cell line (HF19) to gamma-rays (0-40 Gy) at four dose-rates (10, 1, 0.1, and 0.01 Gy/min) at 37 degree C using clamped homogeneous electric field (CHEF) gel electrophoresis. The shape of the dose-response curves, which could be approximated by a straight line over the range 0-20 Gy for irradiation at 4 degree C, became curvilinear when irradiation was carried out at 37 degree C at 10, 1, 0.1, and 0.01 Gy/min and reached a plateau at 10 Gy after irradiation at 0.01 Gy/min. We present a mathematical analysis that predicts the results of irradiation at 37 degree C from dsb induction and repair data obtained at 4 degree C, followed by incubation for repair at 37 degree C. The model assumes that the rate of dsb rejoining changes continuously with repair time and that it is independent of dose and dose-rate in the range 10-40 Gy. The model also assumes a linear induction of dsb with dose at 4 degree C and dsb induction is independent of dose-rate and of temperature during irradiation. Independent measurements of dsb induction at 4 degree C and of repair rate accurately predict the dsb levels after irradiation at 37 degree C, during which both phenomena occur simultaneously.